In polypropylene and some other semi-crystalline polymers, the use of nucleators that enhance the crystallization process is well established. Several types of commercially available nucleators exist such as sodium benzoate, talc, sodium phospate salt (e.g., Na-11, Na-21), N,N-dicyclohexyl-2,6-naphthalenedicarbbis-oxalamide, and bicyclic [2,2,1]heptane di-carboxylate (HPN-68). These typical inorganic and organic salt nucleator additives have high melting points and do not melt or dissolve in polypropylene at typical processing temperatures and as such serve as external nuclei for crystallization.
Also used are “melt sensitive” nucleators (based on modified sorbitols) which not only enhance crystallization but also influence the crystallite sizes. These organic compounds typically have high melting points, are soluble in a polypropylene melt, yet form fibrilar networks during cooling and as such account for more homogeneous nucleation. Because such “melt sensitive” nucleators decrease the crystallite sizes, a substantially reduced haze is the result, thereby extending the application space for semi-crystalline polypropylene (e.g., beverage bottles, food containers, CD cases and other clear packaging).
Because these additives also change the optical properties of the polymer, they may be referred to as clarifiers. Typically, clarifiers are always nucleators but nucleators are not necessarily clarifiers. Currently, prices for a clarifier additive are high and lacking competition, which puts clarified polypropylene (cPP) margins under pressure. Clearly, there is a need for new generations of clarifiers that set new benchmarks for cost and performance.